Field of the Invention
This disclosure relates to force control of a robot arm.
Description of the Related Art
An industrial robot includes a multi-joint robot arm and a robot hand attached to a distal end of the robot arm. In a fitting operation or an operating object acquiring operation using a robot of this type, tolerance of an operating object, installation error, and positional displacement of the operating object being gripped by the robot hand may occur. Accordingly, the positional displacement in a direction intersecting a distal end shaft extending from the robot arm to the robot hand occurs between the operating object and the robot hand.
Therefore, if only position control for controlling angles of respective joints of the robot to control command values is performed, fitting failure or operating object acquiring error may occur due to positional displacement between the operating object and the robot hand. Therefore, the robot includes a force detector configured to detect a fore acting on the robot hand and performs force control of the robot arm with a control device (See Japanese Patent Laid-Open No. 5-69358).
The force control includes trajectory correction control and reached position sensing control. The trajectory correction control includes detecting a contact force generated due to positional displacement between the operating object and the robot hand during fitting or during acquisition of the operating object with the force detector, and correcting a trajectory of the robot arm so that the contact force generated in a direction intersecting a distal end shaft decreases (approaches zero). The reached position detection control includes sensing a reached position in an approaching direction, which is a direction of extension of the distal end shaft, and stopping the robot arm.
In a configuration of the robot including the force detector, an output other than a force to be detected may be added to an output of the force detector as noise due to vibrations during the operation of the robot. When performing the trajectory correction control by using the output from the force detector, the correction may become unstable due to the influence of noise. Therefore, the trajectory correction control is performed by performing low-pass filtering of the output from the force detector.
However, when performing reached position sensing control, if control is performed by filtering the output from the force detector by using a filter having the same characteristics as the case where the trajectory correction control is performed, the force generated upon reaching can be detected with only low sensitivity. Therefore, the timing at which the robot arm is stopped may be delayed. If the timing at which the robot arm is stopped is delayed, the robot hand and the force detector, that is, the robot may stop in an overloaded state.
In contrast, if the features of the filter are set so as to detect a force at a high sensitivity by placing priority on the reached position sensing control, a transient response to a force generated when reaching becomes quicker, so as to prevent the robot from becoming overloaded. However, when performing trajectory correction control, removal of a vibrational component included in a result of force detection is not sufficient. Therefore, the trajectory correction operation becomes unstable.
Accordingly, this disclosure is intended to correct in a stable manner positional displacement of a robot hand in a direction intersecting a distal end shaft, and sense that the robot hand has reached quickly in a direction of extension of the distal end shaft.